The present invention relates to an improved electric power steering apparatus in which a steering force assisting motor is driven and controlled on the basis of torque of a steering wheel detected by a torque sensor and driving of the steering force assisting motor to a reverse direction with respect to the direction of the detected torque is prohibited when the detected torque is within a specified range.
FIG. 1 is a block diagram showing an outline structure of a conventional electric power steering apparatus as already proposed in US Patent Application No. 08/542252 (Japanese Patent Application Laid-Open No. 9-11920 (1997)) by the applicant of the present invention. In this electric power steering apparatus, a detected torque signal indicating torque of a steering wheel detected by a torque sensor 1 is sent to a CPU 2. Upon receipt of this detected torque signal, the CPU 2 reads out a target current corresponding to the detected torque signal from an incorporated target current table. By further differentiating the detected torque signal and adding the obtained value to the target current (PD control), a motor current target value (for volume and direction) for a steering force assisting motor 4 is determined.
The CPU 2 drives the motor 4 by controlling a motor driving circuit 3 on the basis of this motor current target value which is made to be a target value for automatic control. At this point, a driving current of the motor 4 detected by a right-direction driving motor current detecting circuit 5a and a left-direction driving motor current detecting circuit 5b is input into the CPU 2 as a feedback value for the automatic control.
The motor current target value is also sent from the CPU 2 to a driving logic determining circuit 9 and a motor non-driving detecting circuit 11 which will be explained later.
The detected torque signal from the torque sensor 1 is also input into a direction prohibitive range determining circuit 8, and this direction prohibitive range determining circuit 8 is preliminarily stored with specified values for both respective directions of the detected torque as shown in FIG. 2 for informing the driving logic determining circuit 9 and a dead zone detecting circuit 10 whether the detected torque is within the range of these specified values.
Further, the detected torque signal from the torque sensor 1 is also sent to a differentiating circuit 20, and the differential value obtained in the differentiating circuit 20 is sent to a torque change direction detecting circuit 21 to detect a changed direction of the detected torque. A signal indicating the changed direction of the detected torque is sent to an abnormal current determining circuit 6 which will be described later and the driving logic determining circuit 9.
A detecting signal from the dead zone detecting circuit 10 for detecting a dead zone of the detected torque in which the motor current target value should be set to zero is sent to the abnormal current determining circuit 6 for determining whether the detected torque is within the dead zone, the motor driving current is greater than a specified value (e.g., 10 A) and that the direction in which the motor is driven on the basis of the motor driving current and the changed direction of the detected torque are opposite with respect to each other. At this point, the specified value for the motor driving current is set to be greater than a maximum value of a current for returning the steering wheel and, simultaneously, to be a value at which the electric power steering apparatus is not in an unstable condition.
The driving logic determining circuit 9 determines whether the detected torque is outside the dead zone, the direction in which the motor is to be driven on the basis of the motor current target value and the direction of the detected torque are opposite with respect to each other, and the direction in which the motor is to be driven on the basis of the motor current target value and the changed direction of the detected torque are opposite with respect to each other. Each of the results obtained in the driving logic determining circuit 9 and the abnormal current determining circuit 6 is sent to a driving abnormality detecting circuit 7 and when the driving abnormality detecting circuit 7 detects abnormalities in the obtained results, a detecting timer 12 and a deciding timer 15 are started to time their respective set times T1 (e.g. 10 msec) and T2 (e.g. 1 sec).
When the detecting timer 12 completes counting time T1, a latch circuit 13 holds an abnormality detected signal from the driving abnormality detecting circuit 7. While the latch circuit 13 holds the abnormality detected signal from the driving abnormality detecting circuit 7, a diagnosis lamp 19 is switched ON and a motor driving prohibiting circuit 14 stops the operation of the motor driving circuit 3.
The latch circuit 13 is cleared when the motor non-driving detecting circuit 11 for detecting that the motor current target value is zero detects a zero value.
When the deciding timer 15 completes counting time T2, a latch circuit 16 holds the abnormality detected signal from the driving abnormality detecting circuit 7. While the latch circuit 16 holds the abnormality detected signal from the driving abnormality detecting circuit 7, the diagnosis lamp 19 is switched ON and a fail-safe relay 18 for connecting the motor driving circuit 3 and a power source is turned OFF by a relay driving prohibiting circuit 17.
The operation of the electric power steering apparatus of the above-described structure will be now explained.
The detected torque detected by the torque sensor 1 is input into the direction prohibitive range determining circuit 8 whereupon the direction prohibitive range determining circuit 8 determines the direction and volume of the detected torque to send a signal indicating the direction of the detected torque to the driving logic determining circuit 9. As for the volume of the detected torque, an informing signal concerning whether the detected torque is greater or smaller than the specified value (whether they are within or outside the dead zone) in the respective leftward and rightward directions is sent to the dead zone detecting circuit 10 as shown in FIG. 2. From the dead zone detecting circuit 10, an informing signal concerning whether the detected torque is within or outside the dead zone is sent to the abnormal current determining circuit 6.
The detected torque detected by the torque sensor 1 is also input into the differentiating circuit 20 and the calculated value obtained in the differentiating circuit 20 is sent to the torque change direction detecting circuit 21 to detect the changed direction of the detected torque. A signal indicating the changed direction of the detected torque is sent to the abnormal current determining circuit 6 and the driving logic determining circuit 9.
At this point, the driving logic determining circuit 9 informs the driving abnormality detecting circuit 7 of abnormalities during a period in which the detected torque is outside the dead zone, the direction in which the motor is to be driven on the basis of the motor current target value and the direction of the detected torque are opposite with respect to each other, and the direction in which the motor is to be driven on the basis of the motor current target value and the changed direction of the detected torque are opposite with respect to each other.
Further, the abnormal current determining circuit 6 informs the driving abnormality detecting circuit 7 of abnormalities during a period in which the detected torque is within the dead zone, the motor driving current is greater than the specified value, and the direction in which the motor is driven on the basis of the motor driving current and the changed direction of the detected torque are opposite with respect to each other.
Upon receipt of abnormality information from the driving logic determining circuit 9 or the abnormal current determining circuit 6, the driving abnormality detecting circuit 7 detects the abnormality and actuates the detecting timer 12 and the deciding timer 15. When the abnormality information from the driving logic determining circuit 9 or the abnormal current determining circuit 6 is interrupted, the detecting timer 12 and the deciding timer 15 are reset by the driving abnormality detecting circuit 7.
When the detecting timer 12 completes counting of time T1 in the absence of interruptions of abnormality information sent from the driving logic determining circuit 9 or the abnormal current determining circuit 6 to the driving abnormality detecting circuit 7, it allows the latch circuit 13 to hold the abnormality information sent from the driving logic determining circuit 9 or the abnormal current determining circuit 6. While the latch circuit 13 holds the abnormality information, the diagnosis lamp 19 is switched ON and the motor driving prohibiting circuit 14 is actuated to prohibit driving of the motor 4. During this period, when the motor non-driving detecting circuit 11 detects that the motor current target value is zero, the latch circuit 13 is cleared and the actuation of the motor driving prohibiting circuit 14 is terminated to release the driving prohibiting condition of the motor 4.
When the deciding timer 15 completes counting of time T2 in the absence of interruptions of abnormality information sent from the driving logic determining circuit 9 or the abnormal current determining circuit 6 to the driving abnormality detecting circuit 7, it allows the latch circuit 16 to hold the abnormality information sent from the driving logic determining circuit 9 or the abnormal current determining circuit 6. While the latch circuit 16 holds the abnormality information, the diagnosis lamp 19 is switched ON, the relay driving prohibiting circuit 17 is actuated, the fail safe relay 18 is turned OFF, and the power source of the motor driving circuit 3 is turned OFF. The latch circuit 16 is cleared when the power source of the electric power steering apparatus is turned OFF (the ignition key is turned OFF).
Since the latch circuit 13 is reset upon non-driving detection of the motor by the CPU 2, it may be possible that in a runaway condition of the CPU 2 it may become unstable by repeating detecting and returning. Thus, time T2 of the deciding timer 15 is set to be sufficiently longer than time T1 of the detecting timer 12, and in case driving abnormalities should continue during this time T2, occurrence of breakdown is confirmed.
The electric power steering apparatus determines an assist current and steering wheel returning current based on torque signals in the CPU 2 and performs differential control of the detected torque signal (angular speed differential control). In this case, when a large gain is set for the differential control, driving abnormalities may be detected although the CPU 2 is in good order (not in a runaway condition) when a high-flow of the motor driving momentarily occurs even though the direction in which the motor 4 is to be driven on the basis of the motor current target value and the direction of the detected torque are opposite with respect to each other and the detected torque is within the dead zone.
Therefore, the set time Ti for the detecting timer 12 needs to be longer than a time in which the driving abnormality detecting circuit 7 operates in a normal condition; however, it presented a drawback in that a runaway of the CPU 2 may be tolerated during this period when the time is set to be too long, and it could not function as a means for ensuring safety in runaway conditions.
Accordingly, the above-described electric power steering apparatus is provided to comprise the differentiating circuit 20 and the torque change direction detecting circuit 21, wherein an additional condition that the direction in which the motor 4 is driven on the basis of the motor driving current and the changed direction of the detected torque are opposite with respect to each other is included for determining driving abnormalities, and the time T1 is set to be as short as possible. However, this structure cannot cope with the problem in that, when a momentary condition occurs in which the direction in which the motor 4 is to be driven on the basis of the motor driving current target value and the direction of the detected torque are opposite with respect to each other although the CPU 2 is in good order, and the time representing this moment exceeded the short time T1, driving abnormalities (runaway of CPU) is detected.